Osmosis and reverse osmosis in solutions: Monte Carlo simulations and van der Waals one-fluid theory

Osmosis and reverse osmosis in solutions have been studied by computer simulations using the Gibbs ensemble Monte Carlo technique, and by using the equation of state method and the van der Waals one-fluid (vdW1) approximation. Representative solutions have been studied over a wide concentration range, so that both osmosis and reverse osmosis, as well as a transition between the two are observed. Properties investigated include the osmotic pressure, the solvent molecule exchange, and the chemical potential. The results show very clearly that vdW1 is a powerful theory for the systems studied here, and with just a few exceptions was found to be quite accurate. In addition, the osmotic pressure results are compared with the well known van't Hoff approximation, and found to do very well for modelling even quite high concentrations of almost ideal mixtures, but not that well for non-ideal mixing in solutions. Finally, the results show that even in the simpler systems studied, reverse osmosis can take place at a significant level. This confirms the promise of reverse osmosis as an efficient separation technique, as well as the viability of molecular simulation methods to study this important phenomenon.